The answer isn't that easy. You definitely will become frustrated with a very minimal setup, and you can also spend more than needed.

What format do you intend to edit, and do you have some sort of budget? There's also more to a setup than just a computer. You need scratch disks, and they should be fast enough for the format you intend etc... plus external video if you intend to properly color correct or TV consumption etc.

HI Jerry,
Thanks for your reply, I understand purchasing a mac or pc is always a matter of budget and usage. I have about € 1700,- (= $2235,-) to spent and do not need a fullblown fcp 7 version neither, edits i expect to make are not very demanding, simple cuts/dissolves may be some light gfx and text.I am looking for a 21,5 inch 2,7 GHz or a 27 inch 2,7 GHz iMac.
In case I need external harddrives , what kind do you recommend?
Thanks in advance, Ruud.

Buy the fastest iMac you can, and you will need an external hard drive to use with the system for use as a scratch disk. MIght look at the Refurbs that Apple sells in the online store. Never seen a problem with them, and they have full warranty. Click on the "special deals" link in the Apple online store.